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INCREASED CAPACITY R/L CROSSLINK CONCEPT

Publishing Venue

Motorola

Related People

Robert Foncannon: AUTHOR

Abstract

On the current Iridium@ program, satellite to satellite communications is accomplished via four half duplex 12.5 MBPS burst rate K-Band RF crosslinks. Two of these crosslinks point fore and aft (In Plane Links) and two right and left (Cross Plane Links). Transmit and receive timing for all crosslinks are slaved together to a constellation wide time base known as "Iridium@ Time" (IT), which is maintained to keep each Space Vehicle (SV) time base to within +/- 12.5 usec of each other. The basic frame rate for these links is 9 msec (4.5 receive and 4.5 transmit). This timing was selected to allow the maximum possible in plane link capaci- ty. With the nominal spacing between satellites in a plane, there is roughly 13.5 msec of path delay.

Copyright

Motorola Inc. September 1999

Country

United States

Language

English (United States)

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INCREASED CAPACITY R/L CROSSLINK CONCEPT

by Robert Foncannon

On the current Iridium@ program, satellite to satellite communications is accomplished via four half duplex 12.5 MBPS burst rate K-Band RF crosslinks. Two of these crosslinks point fore and aft (In Plane Links) and two right and left (Cross Plane Links). Transmit and receive timing for all crosslinks are slaved together to a constellation wide time base known as "Iridium@ Time" (IT), which is maintained to keep each Space Vehicle (SV) time base to within +/- 12.5 usec of each other. The basic frame rate for these links is 9 msec (4.5 receive and 4.5 transmit). This timing was selected to allow the maximum possible in plane link capaci- ty. With the nominal spacing between satellites in a plane, there is roughly 13.5 msec of path delay.

This is enough delay such that a transmission slot from one satellite falls into a receive slot of its neighbor. All crosslinks on a single satellite must have non-overlapping transmit and receive frames (the receive chain must be blanked during transmit to prevent saturation of the receiver). The limitation with this approach is that the path delay between cross plane liis varies with the range from SV to SV (in a polar orbit such as Iridium@, adjacient planes grow closer together as the satellites near the poles). Total path timing changes by 5.66 msec due to range as the satellites near the poles, causing a complete overlap of transmit slots, reducing the lii capacity to zero (Figure 1, Baseline Timing).

The current concept for crosslink operation maintains full capacity on the Fore/Aft (F/A) links at all times while the Right/Left (R/L) capacity reduces to zero as the satellites approach 68" in lati- tude. There is a simple way to trade F/A capacity for R/L. This enhanced concept allows full capacity for the R/L links at the high latitudes while shutting

down the F/A links. There may be scenarios where this makes sense, depending on the network loads and particularly as a fault mitigation technique for a failed Fore or Aft link.

DESCRIPTION OF TECHNIQUE

Enhanced R/L capacity at high latitudes is made possible by shifting the K-Band frame timing on one satellite's R/L links by 4.5 msec with respect to its neighbor (shown in Figure 1, Enhanced Timing). Since the delay at initial acquisition is 9 msec, this lines up the crosslink frames and provides 100% capacity at the poles. The transmit gates are slaved to the K-Band frame of each satellite (one is shifted

4.5 msec w.r.t. the other), and the receive gates are varied as the range increases to compensate for path delay (Figure 2). The capacity linearly reduces to 50% at 3,375 Km (11.25 msec).

At 3,375 Km, the Right crosslink shifts l/2 of a K-Band frame, effectively removing the 4.5 msec difference. At this point, the receive gate is slaved to the K-Band frame of each satellite, and the trans- mit gates of each SV are allowed to vary to compen- sate for increasing path delay. This...